Recently, image capturing devices have become widely used in portable and non-portable devices such as cameras, mobile phones, webcams and notebooks. These image capturing devices conventionally include an electronic image detector such as a CCD or CMOS sensor, a lens system for projecting an object in a field of view onto the detector and an electronic circuitry for receiving and storing electronic data provided by the detector.
Resolution and optical zoom are two important performance parameters of such image capturing devices.
Resolution of an image capturing device means the minimum distance two point sources in an object plane can have such that the image capturing device is able to distinguish these point sources. The resolution depends on the fact that due to diffraction and aberrations each optical system projects a point source not as a point but as a disc of predetermined width and having a certain light intensity distribution. The response of an optical system to a point light source is known as point spread function (PSF).
The overall resolution of an image capturing device mainly depends on the smaller one of two components: the optical resolution of the optical projection system and the resolution of the detector.
Herein, the optical resolution of an optical projection system shall be defined as the full width at half maximum (FWHM) of its PSF. In other words, the peak values of the light intensity distribution of a projection of two point light sources must be spaced at least by the FWHM of the PSF in order for the image capturing device to be able to distinguish the two point light sources. However, the resolution could also be defined as a different value depending on the PSF, e.g. 70% of the width at half maximum. This definition of the optical resolution might depend on the sensitivity of the detector and the evaluation of the signals received from the detector.
The resolution of the detector is defined herein as the pitch (i.e. distance middle to middle) of two adjacent sensor pixels of the detector.
Optical zoom signifies the capability of the image capturing device to capture a part of the field of view of an original image with better resolution compared with a non-zoomed image. Herein, it is assumed that in conventional image capturing devices the overall resolution is usually limited by the resolution of the detector, i.e. that the FWHM of the PSF can be smaller than the distance between two neighboring sensor pixels. Accordingly, the resolution of the image capturing device can be increased by selecting a partial field of view and increasing the magnification of the optical projection system for this partial field of view.
E.g., ×2 optical zoom refers to a situation where all sensor pixels of the image detector capture half of the image, in each dimension, compared with that of ×1 optical zoom.
The difference between “optical zoom” and “digital zoom” in this document is that applying “digital zoom” merely corresponds to signal interpolation where no additional information is actually provided. “Optical zoom” for that matter includes a magnification of the projected partial image and provides more information and better resolution.
Prior art in realizing optical zoom consists of either changing the distance between the lenses and/or changing the focal length of some of the lenses in a lens module.
One conventional way for obtaining optical zoom is through a mechanical apparatus which effectively changes the magnification of the optical system. This can be achieved by changing the distance of lenses of the optical projection system by mechanically displacing one or more of the lenses while controlling the location of the image plane. However, such conventional image capturing systems require a complex mechanical system including several lenses and a control to drive this lens system. The mechanical system is large, heavy, subject to mechanical failure and expensive.
Alternatively, other prior art techniques for obtaining optical zoom are based on variable focal length lenses. In such embodiments, individual lenses in a lens system are capable of changing their focal length in the presence of an electric field or mechanical pressure. These lenses are typically filled with fluid of one or more types and are capable to change their shape and therefore the lens' focal length. Such solutions typically result in poor image quality in comparison with fixed focal length systems. Moreover, they are often prone to fatigue and aging effects.
The mentioned optical systems typically require moving parts and/or special drivers with possibly high-voltage circuitry and do not result in a cost-effective solution.
It is therefore an object of the present invention to provide an image capturing device having an improved image quality which is adapted to prevent the above drawbacks.
Particularly, it is an object to provide an image capturing device having small size, no or few moving parts and being able to provide increased resolution compared to conventional image capturing devices having the same field of view.